1. Field of the Invention
The present invention relates to a polishing apparatus, and more particularly to a polishing apparatus for polishing a workpiece such as silicon on insulator (SOI), a bare silicon wafer, or an oxide film wafer. The present invention also relates to a method of detecting completion of an initialization process of a polishing pad attached to a polishing table.
2. Description of the Related Art
There has been developed a polishing apparatus for polishing a substrate such as an SOI wafer, a bare silicon wafer, or an oxide film wafer. Such a polishing apparatus includes a polishing table having a polishing pad, such as flexible polyurethane foam, attached to an upper surface of the polishing table, and a top ring for holding and rotating a substrate. The substrate, which is rotated by the top ring, is brought into contact with the polishing pad provided on the rotating polishing table and pressed against the polishing pad. Thus, the substrate is polished by relative movement between the substrate and the polishing pad. When a new polishing pad is attached to the upper surface of the polishing table, an initialization process should be performed for the new polishing pad prior to a polishing process. Specifically, a polishing pad such as a flexible polyurethane foam pad has a protective layer formed as an uppermost layer. Accordingly, an initialization process is required to remove the protective layer before a polishing process.
In a conventional polishing apparatus, a predetermined number of dummy wafers (three to eighty dummy wafers) are polished to initialize a polishing pad. Then, a monitor wafer is polished to measure a polishing rate or precision of a polished surface of the monitor wafer. Thus, completion of the initialization process of the polishing pad is detected based on the measurement results. However, timing of the detection largely depends on the operator's experience. Accordingly, it is impossible to quantitatively determine when the initialization process of the polishing pad is completed. Further, since polishing pads have different qualities, the period of time required for an initialization process varies according to the polishing pads. In consideration of these variations, the initialization process is continued for a period longer than the period required for actual initialization.
Although there has been known a method of detecting an endpoint of a polishing process of a semiconductor wafer, such a method is not employed to detect completion of an initialization process of a polishing pad.